# Class Project! Help Quick!

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##### New Member
I've got to do a class project on Zero-crossing detectors, comparators and Schmitt triggers all using 741 op-amps.
I know all the basics about 741 and also how the above circuits work.
I need to do a project that shows practical applications of the above mentioned topics.

I really hope you'll all come up with something!

P.S. I'm a new member!

#### tcmtech

##### Banned
If its a class project I would bet that the class texts and board notes your instructor gave you covered it!

You just got to be able and willing to read and pay attention in class.

P.S. WE really hope YOU come up with something! WE dont get a grade on it, YOU do!

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##### New Member
Waaaaah!

NOOO!!! Our text books do not cover any practical applications!!!!!!!!
Plus our lecturer didn't teach any basics!!!!!! We study by ourselves!!!!!!!! Just give me some ideas as to where comparators, zero crossing detectors and Schmitt triggers are used... please!!!!

Its ok if you cant draw a circuit for me... just an idea...

##### New Member
Please tell me where are zero crossing detectors, comparators or Schmitt triggers are used... Pracctical applications!!! I know how they work, but I dunno where!!!!!
I've been searching for the past 2 weeks... I didn't get much help from any of my seniors or lecturers... Please help!!!
I promise to write the names of those who help me in my synopsis!
Gosh I'm desperate! It doesn't add to my grades or anything but the lecturer shows me hell if i don't submit!!!
Plus she put me with 3 boys who do nothing to help!!

#### JimB

##### Super Moderator
Don't make multiple posts on the same subject.

OK, a few practical applications.

Comparator.
Say you have an analogue signal which represents the level of liquid in a tank, when the tank is empty the signal is 1volt, when the tank is full the signal is 5volts.
Now, if you want an alarm to warn when the tank is less than 10% full, you may use a comparator to give an output when the signal is less than 1.4volts.

Schmitt trigger.
Our tank level alerm is OK, but when there are ripples on the liquid surface, ie the level is varying slightly, the alarm will go on-off-on-off-on-off...etc as the level varies around 10%.
To cure this we put a bit of positive feedback on the comparator to give it some hysteresis and in effect create a Schmitt trigger.

Zero Crossing detector.
Usually used to detect when AC voltage waveforms are at zero volts.
If we are switching an AC supply using thyristors etc, to minimise switching spikes and RF interference, it is best to switch them when the AC supply is at zerovolts, which it is every half cycle. Use the zero crossing detector to trigger the thyristor.

JimB

PS, did I mention:

Don't make multiple posts on the same subject.

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##### Banned
One application for zero crossing would be to switch a device on (such as a relay) when the voltage on the AC line is zero so that there is no arcing from the contact bounce.

Comparators are useful for many different things. One example would be the comparators that are built into the 555 IC timer. It allows the 555 to generate pulses of various frequencies and lengths in different ways by charging and discharging a capacitor to 1/3 or 2/3's of the VCC voltage, the comparators trigger at the 1/3 and 2/3s level.

Schmitt triggers are used to avoid oscillation if a voltage is RIGHT at the point where a comparator would trigger, such as if there's any noise on the line the comparator might flip on and off very rapidly. It's called hysteresis.

I'm very surprised this stuff isn't explained to you in your course work.

##### New Member
Comparator.
Say you have an analogue signal which represents the level of liquid in a tank, when the tank is empty the signal is 1volt, when the tank is full the signal is 5volts.
Now, if you want an alarm to warn when the tank is less than 10% full, you may use a comparator to give an output when the signal is less than 1.4volts.

Now now what is the connection between the liquid and the voltage? Do I need to add a liquid level indicator or something?
i.e how do I get a signal varying from 1V to 5V as the liquid rises in the tank?

Heyy and thanks a lot Jim!!! I atleast have something to say!!!

#### JimB

##### Super Moderator
Now now what is the connection between the liquid and the voltage? Do I need to add a liquid level indicator or something?
You were asking for a PRACTICAL application, I have just given you one.
I just told you where a comparator may be used.

JimB

##### New Member

We have studied about Hysteresis... Our syllabus doesn't include practical applications at all...
Practical as in circuits in day-to-day applications.
Oh I don't want to complain but our lecturer asks us to write all the circuits all by ourselves!!! She doesn't even teach us the basics!
I was quite happy when she gave me 44 marks out of 50 in my first test!

##### New Member
@JimB

Oh I wanted to know if I've to cascade( if I can use this word) a liquid level indicator before a comparator... I am not clear about how the comparator gets the input signal...

##### Banned
Use your brain Ayaskanta. Seeing as how you know how these devices function now you can imagine their use in any number of ways. It doesn't sound like the teacher is asking for working circuits so just come up with something you think they would be useful for.

#### Torben

##### Well-Known Member
Comparator.
Say you have an analogue signal which represents the level of liquid in a tank, when the tank is empty the signal is 1volt, when the tank is full the signal is 5volts.
Now, if you want an alarm to warn when the tank is less than 10% full, you may use a comparator to give an output when the signal is less than 1.4volts.

Now now what is the connection between the liquid and the voltage? Do I need to add a liquid level indicator or something?
i.e how do I get a signal varying from 1V to 5V as the liquid rises in the tank?
Yes, you'd need some kind of sensor in the tank which would output a voltage which changes relative to how much liquid is in the tank. You could just as easily use, say, a light sensor which changes a voltage depending on how bright the light shining on it is; or a ultrasonic rangefinder sensor which outputs a different voltage depending on how far away its target is.

Torben

##### New Member
Thnx Torben! Thats what I wanted to know... But I don't think I can use sensors etcetera... I've a low cost budget too
But I can use the information to explain myself to my teacher

#### Torben

##### Well-Known Member
Thnx Torben! Thats what I wanted to know... But I don't think I can use sensors etcetera... I've a low cost budget too
But I can use the information to explain myself to my teacher
For a simple test (if you have to build a simple example circuit) you could always use a potentiometer in place of a sensor to provide a changeable voltage. But a simple light sensor is very easy to build using an LDR (also called a CdS cell) but since you only have 2 days I'd say use the potentiometer to start with.

Torben

##### New Member
A potentiometer in the tank??? How is it going to produce a voltage that I can tap onto the comparator input? I'm still in school!!! Please explain!

#### Torben

##### Well-Known Member
A potentiometer in the tank??? How is it going to produce a voltage that I can tap onto the comparator input? I'm still in school!!! Please explain!
No, not in a real tank--you probably don't have time to find or make a tank with a liquid level indicator. Just say "we don't have a real tank here, so for this test, we're going to pretend that the potentiometer represents the liquid level sensor". Then for the test, you turn the potentiometer down to represent a "low" liquid level, and turn it up to represent a "high" liquid level.

Get what I mean?

Torben

##### New Member
haha!

Oh yes!!!!!!!! I get it!!! WOW!!!!!!!!! Thanks!!!!!!! I really appreciate it!!!!!!!!
Great!!! Keep Posting!!!!!!! (Am I using a lot of exclamations here?)
Its 12:45 AM here.. but I'm going to complete writing the synopsis...
Oh My teacher has also given us all a 50 page assignment!

##### New Member
I came up with this idea where I cascade a Schmitt trigger with an integrator circuit. The Schmitt output is the integrator input and the integrator output is fedback as the Schmitt input.
This is nothing but a free running triangular and square wave generator circuit.
The problem is my teacher doesn't think its a day-to-day application. Could any one please help me modify this idea and tell me where the wave generators are used in day-to-day life?

Or any other ideas will be appreciated!

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##### New Member
Could anyone tell me how transistors(functioning as switches) and a zero crossing detector be used together?

Can they be used in railway signalling?
Transistor is OFF during the negative half cycle of an input (which can be detected by the zero crossing detector) otherwise its ON and drives a green LED???

Do I make sense?

#### arunb

##### Member
so now you want a railway signal ???

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